Magnetic data storage or recording apparatus typically uses a magnetic storage or recording medium in which data is stored in tracks. Moreover, the trend in computer technology is to increase the amount and density of data stored in such media. This generally involves narrower and closer tracks to increase the density of tracks- per inch (TPI). For example, high density storage disks typically have track densities of about 500 to 1000 TPI.
As track density increases, a narrow transducer or head is required for writing/reading the data in tracks and, furthermore, more precise tracking is needed to insure writing/reading of the desired track so that data can be retrieved correctly. To this end, it is vital that the transducer be located accurately on a selected track.
The art has recognized the need for accurate head tracking and various approaches to this end, including servo controlled head positioning systems, have been used with varying degrees of success.
In particular, it is important not only that the tracking technique be reliable but it is also highly desirable that it be easy and inexpensive to implement and also that it be able to handle the inevitable slight variations that are usual in data storage apparatus and media commercially available at reasonable prices.
In patent application, Ser. No. 898,527, which was filed on Aug. 21, 1986, and has a common assignee with the instant application, and which is incorporated herein by reference, there is a described magnetic data storage apparatus in which the storage medium, typically a disk, includes a pattern of optical tracks over which is superimposed the magnetic data. The optical pattern is formed on a transparent base material and comprises a large number of lines arranged in concentric rings that are essentially opaque to light radiation, spaced apart by gaps that are essentially transparent to light radiation. The base is over-coated, generally on both sides, with a layer of magnetic recording medium that is thin enough to be substantially light transparent.
The optical pattern in the disk typically is used as a Ronchi grating adapted for cooperation with a reference grating located outside the disk on a radially displaceable read/write transducer assembly that is part of the drive associated with the disk, when the disk is in an operative position. Illumination, provided from a light source on side of the disk, passes through both the Ronchi grating on the disk and the reference grating on the head assembly and is detected.
Movement of the reference grating with the head relative to the Ronchi grating modulates the intensity of the light reaching the detector in a substantially linear manner from track to track so as to provide head position error signal information to a servo system used to position the read/write transducer in the desired alignment with a chosen track of on the disk.
The particular apparatus described, while useful, has room for improvement in performance. In particular, for controlling tracking of magnetic heads on opposite sides of a double sided disk it utilizes a common illumination source along with a common detector that are relatively far apart and so subject to a variety of problems, such as parallax. Moreover, because a common detector is used for tracking control of both sides of the disk, it is not possible to optimize the spatial relationship of the various elements involved in the tracking for each of the two sides being tracked, such as having the detector on the same side as the active read/write head.
This is a particular problem when a replaceable storage medium, such as a flexible or floppy disk, is employed because one of the heads in the drive generally is mounted for movement toward and away from the opposite head between a confronting operative position and a spaced apart loading position to allow disk insertion and withdrawal. Also, the moveable head is typically mounted on a gimbal arrangement which allows it to better orient itself with respect to the flexible recording medium. Given the movable nature of the gimbal mounted head, its magnetic transducer can not be maintained in fixed spaced relation to a single common detector thereby resulting in incorrect error position signals.
Additionally, the signal-to-noise ratio relatively simple detection arrangement used in that system has room for improvement In particular, it would be useful to have a detection scheme that is less vulnerable to roughness and non-uniformities along the edges of the optical pattern, is able to average detection over relatively large areas to minimize the effect of localized non-uniformities or clumping of particles in the thin magnetic recording layers, is able to adjust for some offset between the magnetic center and optical center of the circular tracks, and is more amenable to adjustment to compensate for variations that are may exist among different disks, transducers, and disk drives as the result of being manufactured at different times.
Various other improvements are possible as will appear from the subsequent detailed description of the invention taken in conjunction with the drawings.